The present invention relates generally to electric motors and their manufacture, and more particularly to methods for forming permanent magnets that use rare earth (RE) additives for improved power density of electric motors.
Permanent magnets are used in a variety of devices, including traction electric motors for hybrid and electric vehicles, as well as wind turbines, air conditioning units and other applications where combinations of small volumes and high power densities may be beneficial. Sintered neodymium-iron-boron (Nd—Fe—B) permanent magnets have very good magnetic properties at low temperatures. However, due to the low Curie temperature of the Nd2Fe14B phase in such magnets, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature. There are two common approaches to improving thermal stability and magnetic properties at high temperatures. One is to raise the Curie temperature by adding Cobalt (Co), which is completely soluble in the Nd2Fe14B phase. However, the coercivity of Nd—Fe—B magnets with Co decreases, possibly because of the nucleation sites for reverse domains. The second approach is to add heavy RE elements such as dysprosium (Dy) or terbium (Tb). It is known that the substitution of Dy for Nd or Fe in Nd—Fe—B magnets results in increases of the anisotropic field and the intrinsic coercivity and a decrease of the saturation magnetization. See, for example, C. S. Herget, Metal, Poed. Rep. V. 42, P. 438 (1987); W. Rodewald, J. Less-Common Met., V111, P77 (1985); and D. Plusa, J. J. Wystocki, Less-Common Met. V. 133, P. 231 (1987). It is a common practice to add the heavy RE metals such as Dy or Tb into the mixed metals before melting and alloying.
However, Dy and Tb are very rare and expensive materials. Heavy REs contain only about 2-7 percent Dy, and only a small fraction of the RE mines in the world contain heavy REs. The price of Dy has increased sharply in recent times. Tb, which is needed if higher magnetic properties are required than Dy can provide, is even more expensive than Dy. Furthermore, these metals may be difficult to work with in their relatively pure form, where for example pure Dy is too soft to form into a powder, and is also easily oxidized. While hydrides of Dy can be used to embrittle the material (and therefore make the formation of powder possible), such materials can adversely impact diffusion characteristics and the ability of the material to work at lower temperatures, which in turn may be incompatible with subsequent sintering or related material consolidation efforts. For example, the rapid diffusion of hydrided Dy means that the normally high temperatures associated with sintering (for example, about 1000° C. or more) could not be used for material consolidation, as at this temperature, the extent of the Dy diffusion—and concomitant need for more material to provide ample coverage—would be too great.
Typical magnets for traction electric motors in hybrid cars and trucks contain between about 6 and 10 weight percent Dy to meet the required magnetic properties, while other applications (such as the aforementioned wind turbines and air conditioners, as well as other vehicular configurations (such as motorcycles that may not have as high of an operating temperature environment as their car and truck counterparts) may have lower Dy needs. Assuming the weight of permanent magnet pieces is about 1-1.5 kg per electric motor, and a yield of the machined pieces of typically about 55-65 percent, 2-3 kg of permanent magnets per motor would be required. Moreover, because other industries compete with permanent magnets for limited Dy resources (thereby exacerbating already high costs associated with such materials), reducing the Dy usage in permanent magnets would have a very significant cost impact, as it would for Tb.
Nd—Fe—B permanent magnets can be produced using a powder metallurgy process, which involves making powders with desired chemical composition. A typical powder metallurgy process includes weighing, pressing under a magnetic field, sintering, aging (e.g., about 5 to 30 hrs, at about 500° C. to 1100° C., in vacuum) and machining in order to produce magnet pieces. Additional surface treatments involving phosphating, electroless nickel plating, epoxy coating or the like may also be used.
The ideal microstructure for sintered Nd—Fe—B magnets is Fe14Nd2B grains perfectly isolated by the nonferromagnetic Nd-rich phase made up of a eutectic matrix of mainly Nd plus some Fe4Nd1.1B4 and Fe—Nd phases stabilized by impurities. The addition of Dy or Tb leads to the formation of quite different ternary intergranular phases based on Fe, Nd and Dy or Tb. These phases are located in the grain boundary region and at the surface of the Fe14Nd2B grains.
The microstructures of Nd—Fe—B sintered magnets have been extensively investigated in order to improve the magnetic properties of such magnets composed mainly of the hard-magnetic Nd2Fe14B phase and the nonmagnetic Nd-rich phase. The coercivity is known to be greatly influenced by the morphology of the boundary phases between Nd2Fe14B grains. The magnetic properties of the Nd—Fe—B sintered magnets are degraded when the magnet size is decreased because the machined surface causes nucleation of magnetic reversed domains. Likewise, in their work entitled Improved Magnetic Properties of Small-Sized Magnets and Their Application for DC Brush-less Micro-Motors, Coll. Abstr. Magn. Soc. Jpn. 142 (2005), 25-30), Machida et al. found that the degraded coercivity of small-sized Nd—Fe—B sintered magnets can be improved by surface treating the formed magnet with Dy and Tb-metal vapor sorption so that there is a uniformly distributed coating of Dy or Tb on the outside of the formed magnet. While such approaches are helpful in improving the properties of magnets that have been treated with Dy or Tb, they do so at great expense by utilizing too much of these precious materials.